SUMMARY/ABSTRACT Recent advances have credentialed immunotherapy as a potent anti-cancer treatment modality. However, with exception of high-risk neuroblastoma (NB), immunotherapy has not yet been shown to improve outcomes for children with solid tumors. For patients with NB undergoing myeloablative therapy and autologous hematopoietic stem cell transplantation, subsequent treatment with dinutuximab, which targets disialoganglioside (GD2) on NB cells, improves 2-year event-free survival from 465% to 665%. However, 40% of patients relapse during or after receiving dinutuximab; regimen-related toxicity is high; and 15% of patients have early disease progression and do not receive this therapy. Other immunotherapies include adoptive cell therapy with chimeric antigen receptor (CAR) modified T cells and checkpoint inhibition with monoclonal antibodies (mAbs). CAR T cells have potent activity against pediatric acute lymphoblastic leukemia but have not yet demonstrated activity against solid tumors. Checkpoint inhibition alone (e.g., anti- PD-1 mAb) may not have significant anti-NB activity due to its efficacy correlating with the frequent tumor cell mutations and to the paucity of mutations in NB. Our strategy focuses on cell and mAb based immunotherapies for NB that does not require high-level inherent immunogenicity and builds upon recent advances in our understanding of the tumor cell:immune system interface. We hypothesize that therapeutic gains will be greatest if strategies both enhance cell intrinsic functions of CAR T cells and activated NK (aNK) cells, and overcome cell extrinsic immunosuppression in the tumor microenvironment (TME). Our Specific Aims are to enhance the potency and durability of CAR T cells and aNK cells and to develop regimens that combine enhanced cell therapies with modulation of the immunosuppressive TME. Our Research Strategy/Approach is 1) to enhance the potency and durability of GD2-CARs using targeted mutagenesis of the scFv to prevent antigen-independent tonic signaling and exhaustion originating from the scFv; to test a novel B7-H3-CAR with demonstrated activity; and to add PD-1 blockade to enhance potency and limit exhaustion; 2) to increase trafficking, persistence and potency of aNK cells by combining them with anti-GD2 and anti-B7-H3 mAbs and with a superagonist IL-15; and 3) to overcome immune suppression in the TME with an anti-CD105 mAb to suppress/eliminate mesenchymal stroma and endothelial cells, with a CSF1R inhibitor to eliminate suppressive MDSC and TAMs (collaboration with Project 4), with a TGFBR1 inhibitor to prevent suppression by TGF?1, and with a FAK/ALK inhibitor to overcome FAK-based induction of T cell suppression and exhaustion (collaboration with Project 1). In summary, novel approaches to enhance the potency and durability of adoptive T cell and NK cell therapies will be developed for translation into clinical trials conducted by Core B (NANT consortium) for patients with high-risk NB.